builtin/reflog.c: let parse-options parse subcommands

builtin/reflog.c: let parse-options parse subcommands

'git reflog' parses its subcommands with a couple of if-else if
statements.  parse-options has just learned to parse subcommands, so
let's use that facility instead, with the benefits of shorter code,
and listing subcommands for Bash completion.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>